The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data and for the sensing of externally generated magnetic fields.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Magnetometers and other magnetic sensing devices are also used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds, magnetic signal couplers, magnetic field determiners, and the like. Such devices provide output signals representing the magnetic fields sensed thereby in a variety of situations.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant xe2x80x9cflux closure,xe2x80x9d i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin. Similar xe2x80x9csandwichxe2x80x9d structures are also used in magnetic sensors.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended xe2x80x9csandwichxe2x80x9d structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a xe2x80x9cgiant magnetoresistive effectxe2x80x9d being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
In the ordinary anisotropic magnetoresistive response, varying the difference occurring between the direction of the magnetization vector in a ferromagnetic thin-film and the direction of sensing currents passed through that film leads to varying effective electrical resistance in the film in the direction of the current. The maximum electrical resistance occurs when the magnetization vector in the field and the current direction therein are parallel to one another, while the minimum resistance occurs when they are perpendicular to one another. The total electrical resistance in such a magnetoresistive ferromagnetic film can be shown to be given by a constant value, representing the minimum resistance, plus an additional value depending on the angle between the current direction in the film and the magnetization vector therein. This additional resistance has a magnitude characteristic that follows the square of the cosine of that angle.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film portion with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film. During subsequent operation of the device having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film""s easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In contrast to this arrangement, the resistance in the plane of a ferromagnetic thin-film is isotropic for the giant magnetoresistive effect rather than depending on the direction of the sensing current therethrough as for the anisotropic magnetoresistive effect. The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the xe2x80x9csandwichxe2x80x9d structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the xe2x80x9csandwichxe2x80x9d or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions. Further, the anisotropic magnetoresistive effect in very thin films is considerably reduced from the bulk values therefor in thicker films due to surface scattering, whereas a significant giant magnetoresistive effect is obtained only in very thin films. Nevertheless, the anisotropic magnetoresistive effect remains present in the films used in giant magnetoresistive effect structures.
As indicated above, the giant magnetoresistive effect can be increased by adding further alternate intermediate nonmagnetic and ferromagnetic thin-film layers to extend a xe2x80x9csandwichxe2x80x9d structure into a stacked structure, i.e. a superlattice structure. The giant magnetoresistive effect is sometimes called the xe2x80x9cspin valve effectxe2x80x9d in view of the explanation that a larger fraction of conduction electrons are allowed to move more freely from one ferromagnetic thin-film layer to another if the magnetizations in those layers are parallel than if they are antiparallel or partially antiparallel to thereby result in the magnetization states of the layers acting as sort of a xe2x80x9cvalve.xe2x80x9d
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
A memory cell structure suitable for permitting the storing and retaining of a digital bit of information, and for permitting retrieving same therefrom, has been demonstrated based on a multiple layer xe2x80x9csandwichxe2x80x9d construction in a rectangular solid. This cell has a pair of ferromagnetic layers of equal thickness and area separated by a conductive nonmagnetic layer of the same shape and area parallel to the ferromagnetic layers but of smaller thickness. These ferromagnetic layers are each a composite layer formed of two strata each of a different magnetic material, there being a relatively thin ferromagnetic stratum in each of the composite layers adjacent the nonmagnetic layer and a thicker ferromagnetic stratum in each of the composite layers adjacent the thin ferromagnetic stratum therein. The ferromagnetic material of the thick stratum in one of the composite layers is the same as that in the thin stratum in the other composite layer, and the ferromagnetic material of the thin stratum in the first composite layer is the same as the ferromagnetic material in the thick stratum of the second composite layer. Each of the composite layers is fabricated in the presence of a magnetic field so as to result in having an easy axis parallel to the long sides of the rectangular solid, or alternatively parallel to the short sides of the rectangular solid depending on desired magnetic and electrical resistance characteristics.
Thus, this memory cell structure has a pair of ferromagnetic layers of matching geometries but different magnetic materials in the strata therein to result in one such layer having effectively a greater saturation magnetization and a greater anisotropy field than the other to result in different coercivities in each. In addition, the structure results in a coupling of the magnetization between the two ferromagnetic layers therein due to exchange coupling between them leading to the magnetizations in each paralleling one another in the absence of any applied magnetic fields. As a result, the electrical resistance of the cell along its length versus applied magnetic fields in either direction parallel thereto is represented by two characteristics depending on the magnetization history of the cell. Each of these characteristics exhibits a peak in this resistance for applied longitudinal fields having absolute values that are somewhat greater than zero, one of these characteristics exhibiting its peak for positive applied longitudinal fields and the other characteristic exhibiting its peak for negative applied longitudinal fields. The characteristic followed by the resistance of the cell for relatively small applied longitudinal fields depends on which direction the magnetization is oriented along the easy axis for the one of the two ferromagnetic layers having the larger coercivity. Thus, by setting the magnetization of the layer with the higher coercivity, a bit of digital information can be stored and retained, and the value of that bit can be retrieved without affecting this retention through a determination of which characteristic the resistance follows for a relatively small applied longitudinal field.
Such memory cell behavior for this structure can be modeled by assuming that the ferromagnetic layers therein are each a single magnetic domain so that positioning of the magnetization vectors in the ferromagnetic layers is based on coherent rotation, and that uniaxial anisotropies characterize those layers. The angles of the magnetization vectors in the two ferromagnetic layers with respect to the easy axis in those layers are then found by minimizing the magnetic energy of these anisotropies summed with that due to the applied external fields and to exchange coupling. That total energy per unit volume is then:
ETot=E1+E2+E12=Ku1 sin2 xcex81xe2x88x92Ms1H cos(xcexa8xe2x88x92xcex81)+Ku2 sin2 xcex82xe2x88x92Ms2H cos(xcexa8xe2x88x92xcex82)+A12 cos(xcex81xe2x88x92xcex82).
Here, Ku1 and Ku2 are anisotropy constants, A12 is the exchange constant, Ms1 and Ms2 are the magnetization saturation values, and H is the externally applied field. As indicated above, once the magnetization vectors have taken an angular position with respect to the easy axis of the corresponding layer at a minimum in the above indicated energy, the effective resistance between the ends of the memory cell structure is determined by the net angle between the magnetization vectors in each of these layers.
Because of the assumption of single domain behavior in the ferromagnetic layers, the above equation would seemingly be expected to improve its approximation of the assistant total magnetic energy as the length and width of that memory cell structure decreased toward having submicron dimensions. However, this mode of operation described for providing the two magnetoresistive characteristics based on the history of the layer magnetizations, in depending on the differing anisotropy fields in the two ferromagnetic layers because of the differing materials used therein, becomes less and less reliable as these dimensions decrease. This appears to occur because decreasing the cell dimensions gives rise to larger and larger demagnetizing fields in the two ferromagnetic layers which, at some point, overwhelm the effects of the anisotropy fields so that the above described behavior no longer occurs as described. In addition, the magnetizations of the two ferromagnetic layers rotate together under the influences of externally applied fields at angles with respect to the corresponding easy axis at angular magnitudes much more nearly equal to one another because of the increasing demagnetization fields in these layers as the dimensions thereof decrease. As a result, these ferromagnetic layers are less and less able to have the magnetizations thereof switch directions of orientation independently of one another as the dimensions thereof decrease so that the structure they are in becomes less able to provide the above described memory function in relying on only these ferromagnetic layer anisotropy differences.
An alternative memory cell structure which is more suited to submicron dimensions is a cell of the kind described above exhibiting xe2x80x9cgiant magnetoresistive effectxe2x80x9d but which has the two composite ferromagnetic layers formed of different thicknesses in the thick strata therein. Thus, the thick strata in one might be on the order of 40 xc3x85 while that of the other might be on the order of 55 xc3x85 as an example. In this structure, reducing the size to submicron dimensions uses the shape anisotropy introduced by this thickness difference to provide different switching thresholds for each of the ferromagnetic composite layers in response to externally applied operating magnetic fields. The shape anisotropy leads to the effect of the demagnetizing field of one layer affecting the switching threshold of the other after the former layer has switched its magnetization direction. As a result, the thicker ferromagnetic layer has a magnetization which is fixed in orientation for externally applied operating magnetic fields that are just sufficient to switch the thinner ferromagnetic composite layer but not great enough to switch the magnetization of the thicker ferromagnetic composite layer. In effect, the demagnetizing fields as the device becomes sufficiently small dominate the anisotropy fields that result from the deposition of the ferromagnetic layers in the presence of a magnetic field. Alternatively, one of the ferromagnetic layers could have the magnetization thereof xe2x80x9cpinnedxe2x80x9d in orientation by an antiferromagnetic layer provided thereon rather than by thickening that ferromagnetic layer relative to the other.
In the absence of externally applied operating magnetic field, the two composite ferromagnetic layers have the magnetizations therein pointing in opposite directions, i.e. they are antiparallel to one another, to result in the structure as a whole having relatively small cell demagnetizing fields and small external stray fields to affect the nearby memory cells. The direction of magnetization in the thicker ferromagnetic composite layer is used to store the digital information which can only be changed in direction by externally applied fields great enough to switch magnetization directions in both composite ferromagnetic layers. That is, storing new information in the cell requires that the thicker ferromagnetic layer be capable of having the magnetization direction therein switched to be in accord with the incoming digital data.
Retrieving information from such a memory cell is accomplished by switching the magnetization direction of the thinner ferromagnetic composite layer only as a basis for determining in which direction relative to the thinner layer is the magnetization oriented in the thicker layer. Typically, both such storing and retrieving has meant that there needs to be a pair of external conductors which can coincidentally supply current to result in a field large enough to switch the magnetization of the thicker ferromagnetic composite layer, but with that current in either conductor alone being able to generate fields only sufficient to switch the threshold of the thinner ferromagnetic layer. In some situations, only a single external conductor need be provided for this purpose because the sense current used in retrieving information from the memory cell can provide the coincident current needed with the current in the external conductor to switch the magnetization direction of the thicker ferromagnetic layer.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic layer. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films used in a xe2x80x9csandwichxe2x80x9d structure on either side of an intermediate nonmagnetic layer which ferromagnetic films may be composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on a quantum electrodynamic effect xe2x80x9ctunnelingxe2x80x9d current.
This xe2x80x9ctunnelingxe2x80x9d current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers which are set by their magnetization directions to provide a xe2x80x9cmagnetic valve effectxe2x80x9d. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the xe2x80x9ctunnelingxe2x80x9d current therethrough. In addition, shape anisotropy is used in such a cell to provide different magnetization switching thresholds in the two ferromagnetic layers by forming one of the ferromagnetic layers to be thicker than the other. Such devices may be provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
A xe2x80x9csandwichxe2x80x9d structure for such a memory cell, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the xe2x80x9cmagnetic valve effectxe2x80x9d if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting xe2x80x9cmagnetic valve effectxe2x80x9d can yield a response which can be several times in magnitude greater than that due to the xe2x80x9cgiant magnetoresistive effectxe2x80x9d in a similar sized cell structure.
As stated above, operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film""s easy axis, the state of the cell determining the value of the binary bit being stored therein.
Alternatively, the plurality of cells can be interconnected with manipulation circuitry having a plurality of transistors so that each cell has a selection transistor electrically coupled thereto that selectively substantially permits and prevents current in at least one direction along a current path through that cell. Thus, in effect, the cell is switching circuit selected rather than coincident currents-magnetic fields selected for data storing and retrieving operations.
Similarly, a magnetic field sensor suited for fabrication with dimensions of a few microns or less can be fabricated that provides a suitable response to the presence of external magnetic fields and low power dissipation by substituting an electrical insulator for a conductor in the nonmagnetic layer. This sensor can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the outer magnetic films provided in a xe2x80x9csandwichxe2x80x9d structure on either side of an intermediate nonmagnetic layer which ferromagnetic films may be composite films, but this insulating intermediate nonmagnetic layer again conducts electrical current therethrough based primarily on a quantum electrodynamic effect xe2x80x9ctunnelingxe2x80x9d current.
This xe2x80x9ctunnelingxe2x80x9d current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a xe2x80x9cmagnetic valve effectxe2x80x9d. Such an effect results in an effective resistance, or conductance, characterizing this intermediate layer with respect to the xe2x80x9ctunnelingxe2x80x9d current therethrough.
In these memory cell types, and in magnetic sensors that are to sense rapidly varying magnetic fields such as in magnetic disk memories and magnetic tape storage systems (xe2x80x9cread head sensorxe2x80x9d), a large memory cell device state switching rate or a large sensor material magnetization response rate to external magnetic field changes is desired to allow rapid data manipulation. The maximum operating rate of many magnetically based circuit devices is improved by the use of materials exhibiting magnetic characteristics that include, to some degree, relatively large magnetic saturations, relatively large anisotropy fields, relatively large electrical resistivity, and relatively small hysteresis, or all of these as will be shown below. Should the surface properties of materials exhibiting such properties be amenable to integration with thin-film based magnetoresistive circuit elements, such materials can be used to improve the maximum operating rate of spin dependent tunneling (SDT) devices, giant magnetoresistive (GMR) devices, and anisotropic magnetoresistive (AMR) devices.
SDT devices are very attractive in many applications and the technology and the technology for their design and manufacture is the most likely magnetoresistive circuit device technology to be next commercialized after the technology for the design and manufacture of GMR devices which is now being used to provide such devices in read heads for hard disk drives and in magnetic field sensor products. SDT devices have two main advantages over the state of art GMR devices that have so far been commercialized. One advantage is a factor of 4 to 6 in higher magnetic field-sensitivity, and the other is the orders of magnitude of greater electrical resistance that is easily attainable in devices of small sizes. A typical SDT junction has a magnetoresistive (MR) ratio between magnetic states of 30% to 50% and a saturation magnetic field of about 20 to 30 Oe, leading to a raw field sensitivity of about 1 to 2%/Oe, and a wide range of resistance-area products (RAP) of 102 to 1010 xcexa9-xcexcm2. With on-chip current biasing, a magnetic field sensitivity as high as 7%/Oe has been achieved. The thermal stability has been found to be adequate for integration with integrated circuit electronics in a monolithic integrated circuit chip and for high temperature operations. These superior properties make SDT devices especially attractive for low-field/low-power system applications. They are also attractive for rapid operation applications assuming low RAP SDT junctions can be made reliably. A low junction RAP is necessary as it indicates relatively small parasitic and intrinsic junction capacitance and resistance which would otherwise lead to RC time constant circuit operation delay problems.
Additional examples of rapid operation magnetic thin-film circuit device applications in addition to the hard drive read heads indicated above include galvanic isolators and magnetic memory. The operating rate of these devices is a key performance parameter if manipulations of data are to be performed with sufficient rapidty to accomplish system performance goals in many of these systems. Such devices typically use the magnetoresistance effect, whereby the orientation of the magnetization of a layer or layers within the thin film multilayer structure is determined by measuring the resistance of the structure after selected device operations.
Several types of magnetoresistive structures are commonly used including SDT device structures, GMR spin valve device structures, GMR multilayer device structures, and AMR device structures. In all cases, the resistance changes as the orientation of the magnetizations of one or more magnetic material layers therein rotates. When these magnetoresistive devices are used in an electrical circuit, the electrical signal changes as the magnetization rotates. Thus, the rate of rotation of the magnetization of the magnetic layer or layers involved is directly related to circuit performance. That is, to the extent that the measured resistance of such a device exactly follows the rotation of the magnetization of the involved magnetic material layers therein, the magneto-dynamics of those device layers determine the rate of operation of the device. However, limitations exist in the structure and design of existing magnetic thin-film based magnetoresistive circuit devices insofar as the rate (device operations per second) at which the devices can operate. These limitations are rooted in both the physical material and the mode of magnetic operation that are typically used.
The dynamic response of the magnetization orientation in a magnetic material layer to an externally applied magnetic field is well known to be described satisfactorily by the Landau-Lifshitz-Gilbert (LLG) equation:
dM/dt=xe2x88x92xcex3Mxc3x97H+(xcex1/|M|)Mxc3x97(dM/dt)
where M is the magnetization, H is the total effective magnetic field, xcex3 the gyromagnetic ratio, xcex1 is the damping constant, and x represents the usual vector cross product.
Because the mathematical formulation is identical, it is useful to describe how the dynamic magnetic action is directly analogous to the precessing of the classical mechanics example of a single fixed point spinning top. Just as the spinning axis of the top (axis of angular momentum) precesses about the axis of an external mechanical torque, so does the magnetic moment (axis of angular momentum) of an atom or molecule precess about the axis of an external magnetic torque. The rate of this precession is directly proportional to the torque.
Modified spherical coordinates will be used for the following discussion. In this coordinate system, the xcfx86=0xc2x0 plane corresponds to the Cartesian X-Y plane, and xcfx86=+90xc2x0 and xe2x88x9290xc2x0 correspond to the Cartesian +Z and xe2x88x92Z axes, respectively. In the xcfx86=0xc2x0 plane, xcex8=0xc2x0, +90xc2x0, and +180xc2x0, corresponds to the xe2x88x92X, +Y, and +X axes of the Cartesian system. Because of demagnetizing fields in ferromagnetic thin films, the magnetization lies in the plane of the film (xcfx86=0xc2x0) in steady state situations. Due to device structural considerations, the field components that can be generated in the vicinity of a thin-film device are also primarily in the xcfx86=0xc2x0 plane. Consequently the magnetodynamic response of interest in a ferromagnetic thin-film material based magnetoresistive circuit device to an in-plane externally applied magnetic field is a rotation from one orientation to another, where both orientations are in the xcfx86=0xc2x0 plane. That the starting and ending orientations are in the xcfx86=0xc2x0 plane does not require that the magnetization be in the plane during the non-equilibrium xe2x80x9cswitchingxe2x80x9d times. The actual dynamic response involves small but important rotations out of the plane.
Suppose the magnetization of a thin ferromagnetic film is initially oriented to be at xcex8=0xc2x0 and xcfx86=0xc2x0. At time =0 picoseconds (ps), an in-plane field, Happlied, is provided. A torque is generated by this field with the magnitude Happliedxc3x97M (this is in the first term in the LLG equation above). If Happlied has some component in the xcex8=+90xc2x0 direction, the initial response of the magnetization to the applied torque is a rotation out of the plane (xcfx86 greater than 0). This initial response quickly generates an out-of-plane demagnetizing field in the xe2x88x92xcfx86 direction as free poles accumulate on the top and bottom surface of the film. This out-of-plane demagnetizing field then generates a torque that causes the magnetization to rotate in the plane in the positive xcex8 direction.
Assume, for example, Happlied remains steadily for 1000 ps. Once xcex8=90xc2x0 (at about 100 ps) the torque due to it is 0. For the half-plane 90xc2x0 less than xcex8 less than 270xc2x0, the torque due to Happlied is in the direction opposite it is for the other half-plane. Thus, the magnetization responds by rotating towards the negative xcfx86 direction, generating an out-of-plane demagnetizing field along xcfx86=+90xc2x0, which creates a torque in the xe2x88x92xcex8 direction. The in-plane rotation of xcex8 is reversed by about t=200 ps. The magnetization will oscillate between xcex8=0xc2x0 and 180xc2x0 if Happlied is left on indefinitely. Following up with the above mechanical top analogy, the torque is provided in the mechanical top analogue by gravity or other mechanical forces or both. In the magnetic moment analogue, the torque is provided by magnetic fields. The plane of the thin film is the same as the xe2x80x9cfloorxe2x80x9d for the top analogue. Where gravity causes the top axis to precess about the vertical direction, the force from vertical demagnetizing fields causes the magnetization to precess in the thin-film plane. An externally applied in-plane force on the top would initially cause nutation, not precession. Likewise, the force from an externally applied in-plane magnetic field generates out-of-plane action, not precession.
In most ferromagnetic thin-film based magnetoresistive circuit devices, some magnetic anisotropy is normally present in the thin film. Magnetic anisotropy means that it is energetically favorable for the magnetization to lie in certain directions. A common arrangement is for the xe2x80x9ceasyxe2x80x9d axis (the energetically favorable axis) to lie along xcex8=0xc2x0 and 180xc2x0 and the xe2x80x9chardxe2x80x9d axis (energetically unfavorable) lies along xcex8=90xc2x0 and 270xc2x0. The size of the anisotropy is often measured in terms of the applied field required to overcome it. For instance, if 75 Oe (steady state) is required to saturate the magnetization (cause it to have its maximum magnitude) at xcex8=+90xc2x0 rather than 0xc2x0, then the magnetic anisotropy is 75 Oe. The results of such a measurement are shown in FIG. 1. The saturation magnetization, Msat, and the anisotropy field, Hk, are defined in terms of the response of the magnetization to a field applied along the easy and hard directions of the material. The hard axis data is the linear and non-hysteretic loop. On this plot, the saturation magnetization is the maximum value of magnetization. The anisotropy field, Hk, is the field at which the magnetization first saturates. Thus, one can see that Msat is about 1000 emu/cc, and Hk is about 75 Oe.
This typical kind of anisotropy results in there being two most favorable (stable) states for the magnetization at xcex8=0xc2x0 and 180xc2x0, and xcfx86=0xc2x0. These two magnetization states directly correspond to the two magnetic states of a digital circuit device such as a magnetoresistive memory (MRAM) cell or a digital mode magnetoresistive isolator. The operating rate of these devices is directly related to the time required to cause the magnetization to rotate from one state to the other in a stable fashion.
Setting aside device operation for a bit, consider the previously discussed ferromagnetic thin film with an effective anisotropy of 75 Oe. This anisotropy field adds to any applied field and acts as a xe2x80x9crestoring forcexe2x80x9d to the magnetization when it is not at xcex8=0xc2x0 or 180xc2x0. If instead of leaving Happlied on in its original orientation (xcex8=+90xc2x0), it is reversed at the optimal times (this turns out to be when the magnetization is at xcex8=90xc2x0 and again at 270xc2x0) the magnetization will precess through an arbitrary number of oscillations. Mathematically, it can be shown that this is a xe2x80x9cresonancexe2x80x9d condition, where the driving frequency and internal restoring forces are ideally matched for maximum response.
Assume there is a sinusoidal in-plane driving field at frequency f. Using the LLG equation, when the damping term (second term on the right side, with damping coefficient alpha) is much smaller than the precession term (first term on the right side), the imaginary part of the permeability (4xcfx80dM/dH) of the magnetic material is:
xcexcxe2x80x3(f)xe2x88x9d1/[1xe2x88x92(f/fFMR)]2
where f is the excitation frequency and fFMR the ferromagnetic resonance frequency, or
xe2x80x83fFMR=xcex3[4xcfx80Msxc2x7Hk]1/2
where Ms is the saturation magnetization and Hk is the effective anisotropy field. It is clear that when excitation frequency f approaches fFMR, xcexcxe2x80x3(f) approaches infinity. This is the mathematical way of describing the resonance phenomena. Generally, a higher fFMR leads to higher device operating rates.
The damping factor, xcex1, in the LLG equation is an empirically measured term that takes into account all means of energy transfer from magnetic rotation to other forms. Several physical phenomena play a role in damping. Structural excitations such as phonons and magnons contribute to a larger and lesser degree, respectively. Thermal heating due to eddy currents and interactions with atomic nuclei also contribute.
Eddy currents are also a problem due to their screening of Happlied. Induced eddy currents act to generate a field in the opposite direction of that being applied. These induced fields can partially or completely cancel out the applied field. The eddy current screening effect can be characterized by a parameter called the xe2x80x9cskin depthxe2x80x9d:
xcex={xcfx81/[xcfx80fxcexc]}1/2
where xcfx81 is the material electrical resistivity, f is the frequency of the applied field, and xcexc is the permeability:
xcexc=[4xcfx80Ms/Hk] (hard axis).
The skin depth, xcex, is the depth at which the net field (applied minus eddy current field) is 1/e times that at the surface. The effects of eddy current screening at a given frequency of device operation can best be minimized by increasing xcfx81.
Many present-day magnetoresistive devices are switched in a quasi-static mode. That is, an in-plane magnetic field is applied parallel to the direction of desired resulting magnetization. After enough time, the magnetization will be oriented along this direction since it has the lowest magnetostatic energy. However, the xe2x80x9cpathxe2x80x9d that the magnetization takes during this reorientation is quite complicated, as it probably takes in several cycles of precessional rotation, and/or domain wall motion. If possible, magnetic switching should not involve magnetic domain wall motion, but rather precession that is uniform across the device. Domain walls may exist prior to the attempt at switching, or they may be nucleated at energetically favorable locations in the initial stage of switching. Once initiated, domain wall propagation occurs through the film until equilibrium is reached. At all points, the local magnetic rotation that makes up the domain wall action is described by the same LLG equation that applies to uniform precession. Consequently, domain wall motion can never be faster than uniform precession, and is usually much slower.
Generally when operating in the dynamic switching regime, a material with a higher fFMR will take less time and energy to switch. Consider again the two-state magnetic thin-film based magnetoresistive circuit device. In order to switch the magnetization from xcex8=0xc2x0 to 180xc2x0 at or near the fFMR, special applied field timing is desirable. An in-plane applied field perpendicular to the magnetization (xcex8=+90xc2x0) is needed to initiate the out-of-plane precession, and the resulting vertical demagnetizing field-induced in-plane rotation. As noted above, however, the in-plane applied field must not be left on indefinitely or a continual oscillation between xcex8=0xc2x0 and 180xc2x0 will result. The anisotropy energy and damping coefficient play important roles in this switching action. In an overdamped case, the applied field must be left on until xcex8 greater than 90xc2x0, at which point the anisotropy field will cause the magnetization to stabilize at xcex8=180xc2x0. In the underdamped case, the applied field must be left on until xcex8 approaches 180xc2x0. Here the same field that induces acceleration when the magnetization is at 0 degrees also induces deceleration when the magnetization approaches xcex8=180xc2x0 degrees. It may also be possible to divide the applied field pulse into two parts at the beginning and end, saving some energy. In this case, the first pulse initiates rotation while the second pulse halts it. At increasingly higher frequencies, increasingly larger fields must be applied to induce the magnetization to precess in the allotted time.
A high saturation magnetization and a high magnetic anisotropy field for a magnetic material makes its FMR frequency high, a high material resistivity makes its skin depth large, and reversals of its magnetization direction through primarily rotation makes that magnetization respond fast to a rapidly changing applied magnetic field.
In order to be considered a good soft magnetic material, xcexc needs to be high, regardless of the speed. Also, xcex decreases with f and is the thickness limit (for any given f) for the magnetic material, beyond which any additional material becomes magnetically inactive. Therefore, the greater the value of xcex the better. We see right away that there is a conflicting requirement for Hk in the magnetic material. A high Hk is needed for a high fFMR, but a low Hk is needed for a high permeability xcexc. Therefore, a compromise has to be reached, depending on the specific application for the magnetic material. For very high frequency applications, such as RF devices, a bias field is normally needed to achieve a high fFMR. However, if the induced anisotropy field can be enhanced in the material, there will be a reduced requirement for the biasing. On the other hand, there is always a desire for a magnetic material with a large saturation magnetization and a high resistivity, along with a low loss at the operating frequency.
The present invention provides a ferromagnetic thin-film based magnetoresistive device having a substrate with a major surface on which a first ferromagnetic material based film is supported having electrically conductive, ferromagnetic material nanogranules embedded in an intergranular material of a smaller electrical conductivity first nonmagnetic. The device may have an intermediate layer adjacent the first ferromagnetic material based film and a second film is on the other side of the intermediate layer of a substantially ferromagnetic material. The material in the nanogranules is one of Fe, Co, Cr or alloys thereof, the intergranular material is one of HfO, TaN, HfCN, HfN, HfTa or CrO, the ferromagnetic material of said second film is one of Fe, Co, Ni or alloys thereof, and the intermediate layer is a nonmagnetic electrically conductive or electrically insulative material. The first film is less than 1.0 xcexcm thick.